The primary objective of the Superfund Basic Research Program is to understand the human health and environmental effects associated with hazardous waste sites, and ultimately to devise strategies for remediating such sites in order to minimize public health concerns. The theme of this Program Project is to develop the scientific bases that are necessary to develop biologically-based risk assessments for several chemicals on the National Priorities List. This will be accomplished by (1) identifying critical mechanisms related to the induction of mutations and cancer by these chemicals, (2) establishing whether or not these mechanisms follow linear or nonlinear dose-response relationships between the high doses employed in animal studies and actual or modeled environmental exposures, (3) examining factors related to individual exposure through the use of ultrasensitive biomarkers and the development of personal monitoring devices, (4) determining if sensitive populations exist that are at greater or lesser risk to selected chemicals than the general population, (5) investigating the degradation and fate of chemicals in relevant ecosystems and determining if the products are more or less toxic than the starting materials, (6) evaluating mass transfer phenomena in heterogeneous multiphase subsurface systems and enhanced methods of remediating such systems, and (7) providing more meaningful estimates of human and ecological exposure to contaminants using stochastic analysis of flow and transport phenomena. The theme addresses many of the stated goals of the Superfund Basic Research Program. We will accomplish our task through the investigations proposed in seven research projects and five supporting cores.